Because of their appreciation for beauty, people in different historical periods have developed various colorful pigments and dyestuffs to beautify life. Luster pigments originated from pearls and then pearl luster pigments were developed. The early pearl pigments consisted of guanine from natural fish scales. However, because of a limited amount and high price of natural pearl pigments, people have begun switching to synthetic pearl luster pigments which fall into two major forms: one is a flake-like inorganic mixture of bismuth oxychloride and calcium-sodium silicates; the other is a multilayer pearl luster pigment formed by coating metal oxides and non-metal oxides on the surface of a natural white mica and synthetic mica with methods such as evaporating and depositing, co-ion sputtering, and wet chemical methods. The reasons for why pearl luster pigments can produce pearl luster effects is that some of incident light is reflected, some is refracted, some is absorbed, and some is refracted and reflected multiple times, which causes the formation of various interference colors and a pearl or rainbow-like effect.
The multilayered pearl luster pigments on a mica base cause the reflected or refracted light to exhibit different types of luster depending on the different grain diameters of the micas; for instance, small grain diameters can produce silk-like luster, while large grain diameters can produce glittering luster.
Because of the difference in thickness of the coating metal oxides, many interference colors are generated, such as, e.g., silver-white, yellow, orange, red, purple, blue and green, all of which are monochromatic.
The crystal-like synthetic calcium-sodium silicate is not applied in many cases as the major pigment ingredient and is often used in small amounts. Because of its low weather resistance and large specific gravity, the application range of the crystal-like synthetic bismuth oxychloride is limited.
The multilayered mica-based pearl luster pigment has excellent weather and chemical resistance because of metal oxides having a high refractive index, such as titanium oxide and/or ferric oxide, coated on the mica surface. In addition, pigments with different grain diameters can be produced through size classification. Therefore, this kind of pigment is widely used in car coating, paints, coating, cosmetics, special wrapping paper, decorative paper, plastics, plastic color agglomerates, artificial leather and various printing inks, etc. However, this pigment causes monochromatic light interference only and does not produce color-shifting at various angles. It is only double-colored, if used as a dye, and thus cannot be used in certain fields and for certain applications.
At present, only such companies as Warck, BSF, and Merck in Germany, and Flex in the United States can produce pigments having color-shifting effects. An analysis of the advantages and disadvantages of their patents is made hereinbelow.
Flex in the U.S. adopts opaque tinsel or metal oxides as the base materials and coats the tinsel alternatively with metal oxides of high and low refractive indexes on one or both sides of the tinsel with vacuum-film plating. By controlling the optical thickness of the coating metal oxide layers, different color changes and color-shift zones can be obtained. The base is stripped from the net with certain meshes or dissolvable films. Therefore, precision instruments, strict technological processes, and highly pure chemical materials are needed resulting in high production cost and low productions. In addition, it is difficult to control the colors and color-shift zones, and a wide use of the pigment is limited because of its high price.
BSF in Germany adopts aluminum powders or synthetic ferric oxides (made under high temperature and pressure) as the base materials, the surface of which is coated alternatively with metal oxides of high and low refractive indexes using the VCD method. An inert gas transports boiling metal particles and deposits them on the surface of the base material at a desired coating thickness. However, there are problems with effective dispersing of the base in the cauldron and with dosing the amounts of the materials. Moreover, there is also the environmental problem of using organic metal compounds. In addition, the pigments are expensive and limited in application because of complicated preparation procedures, the necessity of precision instruments, the high cost of base making, the need for strict procedure control, and the high price of the raw materials.
Merck in Germany adopts synthetic silicon dioxide as the base material (made by plating hydrate glass with certain density on a special instrument and drying and stripping it), and hydrolyses dissolvable inorganic metal and nonmetal compounds to produce hydrate metal oxide having high and low refractive indexes which are coated alternatively on the base material. Because this is a wet chemical procedure, it is easy to operate and control the optical thickness of the desired coating. In addition, it is possible to obtain color-shifting effects by plating a layer of metal oxide having a high refractive index on the silicon dioxide base surface. However, the thickness of the base must be less than 1 micron, usually between 200 and 500 nm, which causes difficulty for the making of the base, high cost, low production, and the need for high precision instruments. Besides, the process of plating, drying, and stripping causes great wear and tear of the production machines and makes the overall procedure complicated so as to result in a high cost of base making. Consequently, the pigment is very expensive, which limits its application.
Warck in Germany adopts organic polymerized silicon liquid crystals of certain sizes, grinding them and sifting them out to pigments. As liquid crystals, the pigments have a short life span and can not be used in certain applications. In addition, the high price of liquid crystals limits their application.
U.S. Pat. No. 6,656,259 describes pigments exhibiting color-shifting effects and their synthesis. The pigments adopt flake-like silicon dioxide as the base material whose surface is coated with single-layered or multi-layered transparent or semi-transparent metal oxides.
U.S. Pat. No. 6,692,561 discloses multi-layered pigments exhibiting strong optical interference and having mica as the base material. The patent alleges that the color of the pigments changes with the viewing angle and that the pigments glitter with interfering lights. The first layer coated on the mica base is a metal oxide with a low refractive index, but the optimal value of its size and its technical specification are not stated. Therefore, it is alleged that the invention does not solve the problem that it sets out to solve. Based on a statistical analysis, the specification disclosed wavelengths at a fixed viewing angle. As a result, the range of color-shifting of the pigment is limited, and the pigment belongs to the group of ordinary pearl luster pigments, not reaching the full effect of color-shifting at different viewing angles, for such pigment should be ascertained by multiangle photometer at least five times.